This invention relates to cement slurries utilized in subterranean wells. One aspect of the invention relates to cement slurries comprising cement, water, and a novel fluid loss additive. A further aspect of the invention relates to a fluid loss additive preferably comprising polyvinylpyrrolidone, carboxymethylhydroxyethyl cellulose, and the sodium salt of naphthalene sulfonate.
In the drilling, completion and servicing of subterranean wells, it is customary to utilize cement or concrete slurries for the purpose of sealing various portions of the well bore, for anchoring well pipe or well casing therein, and for other well known and established purposes. The cement slurry is usually pumped into the well bore under considerable pressure, where it is further subjected to the hydrostatic pressure of the column of fluid which may extend for several thousand feet. The slurry is also subjected to elevated temperatures that usually increase in proportion to the depth of the well.
Because of the widely varying soil formations and operating conditions that may be encountered in drilling and completing different wells, or even at different stages within a single well, additives are frequently utilized in cement slurries in order to achieve certain desired properties. For example, accelerators or retarders are often added to affect the setting time of the cement, thereby varying its ultimate hardness or strength. Fluid loss additives are also frequently employed in cement slurries when working within permeable formations or water sensitive zones where fluid loss control is necessary. However, conventional fluid loss additives have exhibited numerous disadvantages. Many conventional fluid loss additives are loaded with cellulose type materials and tend to greatly increase the viscosities of the slurries in which they are employed. Others are strong retarders and hinder the development of good early compressive strengths. Conventional fluid loss additives representative of those described in the prior art are disclosed in U.S. Pat. No. 2,247,683 to Ludwig, U.S. Pat. No. 3,132,693 to Weisen, and U.S. Pat. No. 3,140,269 to Wahl, et al.
An effective fluid loss additive is therefore needed that can maintain the viscosity of a cement slurry at a level comparable to that of neat cement. Furthermore, cement slurries containing the improved fluid loss additive should be readily pumpable across weak, sensitive zones where slurry fluid loss must be controlled or where annular space is restricted, and should be able to develop high early compressive strengths at bottom hole circulating temperatures as high as 215.degree. F. (102.degree. C.) or above.